Method for transmitting sounding reference signal in wireless communication system supporting unlicensed band, and apparatus supporting same

ABSTRACT

Disclosed are a method for transmitting, by a terminal, a sounding reference signal to a base station without a physical uplink shard channel in a licensed assisted access (LAA) system in which a base station or a terminal performs listen-before-talk (LBT)-based signal transmission, and an apparatus supporting the same.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/678,794, filed on Nov. 8, 2019, which is a continuation of U.S.patent application Ser. No. 16/252,492, filed on Jan. 18, 2019, now U.S.Pat. No. 10,660,138, which is a continuation of U.S. patent applicationSer. No. 16/096,661, filed on Oct. 25, 2018, which is the National Stagefiling under 35 U.S.C. 371 of International Application No.PCT/KR2017/004790, filed on May 10, 2017, which claims the benefit ofU.S. Provisional Application Nos. 62/334,381, filed on May 10, 2016,62/370,719, filed on Aug. 4, 2016 and 62/373,345, filed on Aug. 10,2016, the contents of which are all hereby incorporated by referenceherein in their entirety.

TECHNICAL FIELD

Following description relates to a wireless communication systemsupporting an unlicensed band, and more particularly, to a method oftransmitting a sounding reference signal without transmitting a physicaluplink shared channel in a wireless communication system supporting anunlicensed band and apparatuses supporting the same.

BACKGROUND ART

Wireless access systems have been widely deployed to provide varioustypes of communication services such as voice or data. In general, awireless access system is a multiple access system that supportscommunication of multiple users by sharing available system resources (abandwidth, transmission power, etc.) among them. For example, multipleaccess systems include a Code Division Multiple Access (CDMA) system, aFrequency Division Multiple Access (FDMA) system, a Time DivisionMultiple Access (TDMA) system, an Orthogonal Frequency Division MultipleAccess (OFDMA) system, and a Single Carrier Frequency Division MultipleAccess (SC-FDMA) system.

DISCLOSURE OF THE INVENTION Technical Task

When a terminal transmits a sounding reference signal (SRS) notincluding transmission of a physical uplink shared channel (PUSCH), anobject of the present invention is to newly define LBT(Listen-Before-Talk) performed by the terminal based on a characteristicof an unlicensed band and provide a method for a terminal to transmit anSRS based on the newly defined LBT.

In particular, an object of the present invention is to provide a methodfor a terminal to more reliably transmit an SRS by newly defining an LBToperation for transmitting the SRS only without PUSCH.

It will be appreciated by persons skilled in the art that the objectsthat could be achieved with the present disclosure are not limited towhat has been particularly described hereinabove and the above and otherobjects that the present disclosure could achieve will be more clearlyunderstood from the following detailed description.

Technical Solution

The present invention provides a method for a user equipment to transmita sounding reference signal to a base station in a wirelesscommunication system supporting an unlicensed band and apparatusessupporting the same.

To achieve these and other advantages and in accordance with the purposeof the present invention, as embodied and broadly described, accordingto one embodiment, a method of transmitting an sounding reference signal(SRS), which is transmitted by a user equipment to a base station in awireless communication system supporting an unlicensed band, includesperforming LBT (Listen-Before-Talk) for transmitting an SRS and, if theLBT succeeds, transmitting the SRS. In this case, if the SRStransmission does not include physical uplink shared channel (PUSCH)transmission, the LBT may correspond to random backoff-based LBT basedon a predetermined channel access priority class.

To further achieve these and other advantages and in accordance with thepurpose of the present invention, according to a different embodiment, auser equipment transmitting an sounding reference signal to a basestation in a wireless communication system supporting an unlicensed bandincludes a transmitting unit and a processor configured to operate in amanner of being connected with the transmitting unit, the processorconfigured to perform LBT (Listen-Before-Talk) for transmitting an SRS,the processor, if the LBT succeeds, configured to transmit the SRS. Inthis case, if the SRS transmission does not include physical uplinkshared channel (PUSCH) transmission, the LBT may correspond to randombackoff-based LBT based on a predetermined channel access priorityclass.

In this case, the SRS can be transmitted in the last symbol in timedimension among symbols included in a subframe.

The LBT may correspond to random backoff-based LBT based on a channelaccess priority class having a smallest contention window size among aplurality of channel access priority classes.

For example, the LBT may correspond to random backoff-based LBT based ona channel access priority class having a value of {3,7} as an allowedcontention window size.

A value selected from among 3 and 7 can be applied as a contentionwindow size applied to the LBT for transmitting the SRS according towhether a contention window size recently applied to LBT fortransmitting PUSCH corresponds to a minimum contention window size or anincreased contention window size.

Specifically, when the contention window size recently applied to theLBT for transmitting the PUSCH corresponds to the minimum contentionwindow size, 3 is applied as the contention window size applied to theLBT for transmitting the SRS and when the contention window sizerecently applied to the LBT for transmitting the PUSCH corresponds tothe increased window size, 7 can be applied as the contention windowsize applied to the LBT for transmitting the SRS.

Technical solutions obtainable from the present invention arenon-limited the above-mentioned technical solutions. And, otherunmentioned technical solutions can be clearly understood from thefollowing description by those having ordinary skill in the technicalfield to which the present invention pertains.

Advantageous Effects

As is apparent from the above description, the embodiments of thepresent disclosure have the following effects.

According to the present invention, when an SRS is transmitted onlywithout a PUSCH in a wireless access system supporting an unlicensedband, a terminal can more reliably transmit an SRS via an unlicensedband.

Effects obtainable from the present invention are non-limited by theabove mentioned effect. And, other unmentioned effects can be clearlyunderstood from the following description by those having ordinary skillin the technical field to which the present invention pertains. That is,effects which are not intended by the present invention may be derivedby those skilled in the art from the embodiments of the presentinvention.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention, provide embodiments of the presentinvention together with detail explanation. Yet, a technicalcharacteristic of the present invention is not limited to a specificdrawing. Characteristics disclosed in each of the drawings are combinedwith each other to configure a new embodiment. Reference numerals ineach drawing correspond to structural elements.

FIG. 1 is a diagram illustrating physical channels and a signaltransmission method using the physical channels;

FIG. 2 is a diagram illustrating exemplary radio frame structures;

FIG. 3 is a diagram illustrating an exemplary resource grid for theduration of a downlink slot;

FIG. 4 is a diagram illustrating an exemplary structure of an uplinksubframe;

FIG. 5 is a diagram illustrating an exemplary structure of a downlinksubframe;

FIG. 6 is a diagram illustrating an exemplary CA environment supportedin an LTE-Unlicensed (LTE-U) system;

FIG. 7 is a diagram illustrating an exemplary Frame Based Equipment(FBE) operation as one of Listen-Before-Talk (LBT) operations;

FIG. 8 is a block diagram illustrating the FBE operation;

FIG. 9 is a diagram illustrating an exemplary Load Based Equipment (LBE)operation as one of the LBT operations;

FIG. 10 is a diagram for explaining methods of transmitting a DRSsupported in an LAA system;

FIG. 11 is a flowchart for explaining CAP and CWA;

FIG. 12 is a diagram illustrating a partial TTI or a partial subframeapplicable to the present invention;

FIG. 13 is a diagram for explaining an operation of a UE when the UE isscheduled to transmit PUSCH in an SF #n+1 after the UE transmits an SRSin the last symbol of an SF #n according to one embodiment of thepresent invention;

FIG. 14 is a diagram illustrating configurations of a UE and a basestation in which proposed embodiments are implementable.

BEST MODE Mode for Invention

The embodiments of the present disclosure described below arecombinations of elements and features of the present disclosure inspecific forms. The elements or features may be considered selectiveunless otherwise mentioned. Each element or feature may be practicedwithout being combined with other elements or features. Further, anembodiment of the present disclosure may be constructed by combiningparts of the elements and/or features. Operation orders described inembodiments of the present disclosure may be rearranged. Someconstructions or elements of any one embodiment may be included inanother embodiment and may be replaced with corresponding constructionsor features of another embodiment.

In the description of the attached drawings, a detailed description ofknown procedures or steps of the present disclosure will be avoided lestit should obscure the subject matter of the present disclosure. Inaddition, procedures or steps that could be understood to those skilledin the art will not be described either.

Throughout the specification, when a certain portion “includes” or“comprises” a certain component, this indicates that other componentsare not excluded and may be further included unless otherwise noted. Theterms “unit”, “-or/er” and “module” described in the specificationindicate a unit for processing at least one function or operation, whichmay be implemented by hardware, software or a combination thereof. Inaddition, the terms “a or an”, “one”, “the” etc. may include a singularrepresentation and a plural representation in the context of the presentdisclosure (more particularly, in the context of the following claims)unless indicated otherwise in the specification or unless contextclearly indicates otherwise.

In the embodiments of the present disclosure, a description is mainlymade of a data transmission and reception relationship between a BaseStation (BS) and a User Equipment (UE). A BS refers to a terminal nodeof a network, which directly communicates with a UE. A specificoperation described as being performed by the BS may be performed by anupper node of the BS.

Namely, it is apparent that, in a network comprised of a plurality ofnetwork nodes including a BS, various operations performed forcommunication with a UE may be performed by the BS, or network nodesother than the BS. The term ‘BS’ may be replaced with a fixed station, aNode B, an evolved Node B (eNode B or eNB), an Advanced Base Station(ABS), an access point, etc.

In the embodiments of the present disclosure, the term terminal may bereplaced with a UE, a Mobile Station (MS), a Subscriber Station (SS), aMobile Subscriber Station (MSS), a mobile terminal, an Advanced MobileStation (AMS), etc.

A transmission end is a fixed and/or mobile node that provides a dataservice or a voice service and a reception end is a fixed and/or mobilenode that receives a data service or a voice service. Therefore, a UEmay serve as a transmission end and a BS may serve as a reception end,on an UpLink (UL). Likewise, the UE may serve as a reception end and theBS may serve as a transmission end, on a DownLink (DL).

The embodiments of the present disclosure may be supported by standardspecifications disclosed for at least one of wireless access systemsincluding an Institute of Electrical and Electronics Engineers (IEEE)802.xx system, a 3rd Generation Partnership Project (3GPP) system, a3GPP Long Term Evolution (LTE) system, and a 3GPP2 system. Inparticular, the embodiments of the present disclosure may be supportedby the standard specifications, 3GPP TS 36.211, 3GPP TS 36.212, 3GPP TS36.213, 3GPP TS 36.321 and 3GPP TS 36.331. That is, the steps or parts,which are not described to clearly reveal the technical idea of thepresent disclosure, in the embodiments of the present disclosure may beexplained by the above standard specifications. All terms used in theembodiments of the present disclosure may be explained by the standardspecifications.

Reference will now be made in detail to the embodiments of the presentdisclosure with reference to the accompanying drawings. The detaileddescription, which will be given below with reference to theaccompanying drawings, is intended to explain exemplary embodiments ofthe present disclosure, rather than to show the only embodiments thatcan be implemented according to the disclosure.

The following detailed description includes specific terms in order toprovide a thorough understanding of the present disclosure. However, itwill be apparent to those skilled in the art that the specific terms maybe replaced with other terms without departing the technical spirit andscope of the present disclosure.

For example, the term, TxOP may be used interchangeably withtransmission period or Reserved Resource Period (RRP) in the same sense.Further, a Listen-Before-Talk (LBT) procedure may be performed for thesame purpose as a carrier sensing procedure for determining whether achannel state is idle or busy, CCA (Clear Channel Assessment), and CAP(Channel Access Procedure).

Hereinafter, 3GPP LTE/LTE-A systems are explained, which are examples ofwireless access systems.

The embodiments of the present disclosure can be applied to variouswireless access systems such as Code Division Multiple Access (CDMA),Frequency Division Multiple Access (FDMA), Time Division Multiple Access(TDMA), Orthogonal Frequency Division Multiple Access (OFDMA), SingleCarrier Frequency Division Multiple Access (SC-FDMA), etc.

CDMA may be implemented as a radio technology such as UniversalTerrestrial Radio Access (UTRA) or CDMA2000. TDMA may be implemented asa radio technology such as Global System for Mobile communications(GSM)/General packet Radio Service (GPRS)/Enhanced Data Rates for GSMEvolution (EDGE). OFDMA may be implemented as a radio technology such asIEEE 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, Evolved UTRA(E-UTRA), etc.

UTRA is a part of Universal Mobile Telecommunications System (UMTS).3GPP LTE is a part of Evolved UMTS (E-UMTS) using E-UTRA, adopting OFDMAfor DL and SC-FDMA for UL. LTE-Advanced (LTE-A) is an evolution of 3GPPLTE. While the embodiments of the present disclosure are described inthe context of a 3GPP LTE/LTE-A system in order to clarify the technicalfeatures of the present disclosure, the present disclosure is alsoapplicable to an IEEE 802.16e/m system, etc.

1. 3GPP LTE/LTE-A System

In a wireless access system, a UE receives information from an eNB on aDL and transmits information to the eNB on a UL. The informationtransmitted and received between the UE and the eNB includes generaldata information and various types of control information. There aremany physical channels according to the types/usages of informationtransmitted and received between the eNB and the UE.

FIG. 1 illustrates physical channels and a general signal transmissionmethod using the physical channels, which may be used in embodiments ofthe present disclosure.

When a UE is powered on or enters a new cell, the UE performs initialcell search (S11). The initial cell search involves acquisition ofsynchronization to an eNB. Specifically, the UE synchronizes its timingto the eNB and acquires information such as a cell Identifier (ID) byreceiving a Primary Synchronization Channel (P-SCH) and a SecondarySynchronization Channel (S-SCH) from the eNB.

Then the UE may acquire information broadcast in the cell by receiving aPhysical Broadcast Channel (PBCH) from the eNB.

During the initial cell search, the UE may monitor a DL channel state byreceiving a Downlink Reference Signal (DL RS).

After the initial cell search, the UE may acquire more detailed systeminformation by receiving a Physical Downlink Control Channel (PDCCH) andreceiving a Physical Downlink Shared Channel (PDSCH) based oninformation of the PDCCH (S12).

To complete connection to the eNB, the UE may perform a random accessprocedure with the eNB (S13 to S16). In the random access procedure, theUE may transmit a preamble on a Physical Random Access Channel (PRACH)(S13) and may receive a PDCCH and a PDSCH associated with the PDCCH(S14). In the case of contention-based random access, the UE mayadditionally perform a contention resolution procedure includingtransmission of an additional PRACH (S15) and reception of a PDCCHsignal and a PDSCH signal corresponding to the PDCCH signal (S16).

After the above procedure, the UE may receive a PDCCH and/or a PDSCHfrom the eNB (S17) and transmit a Physical Uplink Shared Channel (PUSCH)and/or a Physical Uplink Control Channel (PUCCH) to the eNB (S18), in ageneral UL/DL signal transmission procedure.

Control information that the UE transmits to the eNB is genericallycalled Uplink Control Information (UCI). The UCI includes a HybridAutomatic Repeat and reQuest Acknowledgement/Negative Acknowledgement(HARQ-ACK/NACK), a Scheduling Request (SR), a Channel Quality Indicator(CQI), a Precoding Matrix Index (PMI), a Rank Indicator (RI), etc.

In the LTE system, UCI is generally transmitted on a PUCCH periodically.However, if control information and traffic data should be transmittedsimultaneously, the control information and traffic data may betransmitted on a PUSCH. In addition, the UCI may be transmittedaperiodically on the PUSCH, upon receipt of a request/command from anetwork.

FIG. 2 illustrates exemplary radio frame structures used in embodimentsof the present disclosure.

FIG. 2(a) illustrates frame structure type 1. Frame structure type 1 isapplicable to both a full Frequency Division Duplex (FDD) system and ahalf FDD system.

One radio frame is 10 ms (Tf=307200·Ts) long, including equal-sized 20slots indexed from 0 to 19. Each slot is 0.5 ms (Tslot=15360·Ts) long.One subframe includes two successive slots. An ith subframe includes2ith and (2i+1)th slots. That is, a radio frame includes 10 subframes. Atime required for transmitting one subframe is defined as a TransmissionTime Interval (TTI). Ts is a sampling time given as Ts=1/(15kHz×2048)=3.2552×10−8 (about 33 ns). One slot includes a plurality ofOrthogonal Frequency Division Multiplexing (OFDM) symbols or SC-FDMAsymbols in the time domain by a plurality of Resource Blocks (RBs) inthe frequency domain.

A slot includes a plurality of OFDM symbols in the time domain. SinceOFDMA is adopted for DL in the 3GPP LTE system, one OFDM symbolrepresents one symbol period. An OFDM symbol may be called an SC-FDMAsymbol or symbol period. An RB is a resource allocation unit including aplurality of contiguous subcarriers in one slot.

In a full FDD system, each of 10 subframes may be used simultaneouslyfor DL transmission and UL transmission during a 10-ms duration. The DLtransmission and the UL transmission are distinguished by frequency. Onthe other hand, a UE cannot perform transmission and receptionsimultaneously in a half FDD system.

The above radio frame structure is purely exemplary. Thus, the number ofsubframes in a radio frame, the number of slots in a subframe, and thenumber of OFDM symbols in a slot may be changed.

FIG. 2(b) illustrates frame structure type 2. Frame structure type 2 isapplied to a Time Division Duplex (TDD) system. One radio frame is 10 ms(Tf=307200·Ts) long, including two half-frames each having a length of 5ms (=153600·Ts) long. Each half-frame includes five subframes each being1 ms (=30720·Ts) long. An ith subframe includes 2ith and (2i+1)th slotseach having a length of 0.5 ms (Tslot=15360·Ts). Ts is a sampling timegiven as Ts=1/(15 kHz×2048)=3.2552×10-8 (about 33 ns).

A type-2 frame includes a special subframe having three fields, DownlinkPilot Time Slot (DwPTS), Guard Period (GP), and Uplink Pilot Time Slot(UpPTS). The DwPTS is used for initial cell search, synchronization, orchannel estimation at a UE, and the UpPTS is used for channel estimationand UL transmission synchronization with a UE at an eNB. The GP is usedto cancel UL interference between a UL and a DL, caused by themulti-path delay of a DL signal.

[Table 1] below lists special subframe configurations (DwPTS/GP/UpPTSlengths).

TABLE 1 Normal cyclic prefix in downlink Extended cyclic prefix indownlink Special UpPTS UpPTS subframe Normal cyclic Extended cyclicNormal cyclic Extended cyclic configuration DwPTS prefix in uplinkprefix in uplink DwPTS prefix in uplink prefix in uplink 0  6592 · T_(s)2192 · T_(s) 2560 · T_(s)  7680 · T_(s) 2192 · T_(s) 2560 · T_(s) 119760 · T_(s) 20480 · T_(s) 2 21952 · T_(s) 223040 · T_(s)  3 24144 ·T_(s) 25600 · T_(s) 4 26336 · T_(s)  7680 · T_(s) 5  6592 · T_(s) 20480· T_(s) 4384 · T_(s) 5120 · T_(s) 6 19760 · T_(s) 23040 · T_(s) 7 21952· T_(s) 4384 · T_(s) 5120 · T_(s) 12800 · T_(s) 8 24144 · T_(s) — — — 913168 · T_(s) — — —

FIG. 3 illustrates an exemplary structure of a DL resource grid for theduration of one DL slot, which may be used in embodiments of the presentdisclosure.

Referring to FIG. 3, a DL slot includes a plurality of OFDM symbols inthe time domain. One DL slot includes 7 OFDM symbols in the time domainand an RB includes 12 subcarriers in the frequency domain, to which thepresent disclosure is not limited.

Each element of the resource grid is referred to as a Resource Element(RE). An RB includes 12×7 REs. The number of RBs in a DL slot, NDLdepends on a DL transmission bandwidth. A structure of an uplink slotmay be identical to a structure of a downlink slot.

FIG. 4 illustrates a structure of a UL subframe which may be used inembodiments of the present disclosure.

Referring to FIG. 4, a UL subframe may be divided into a control regionand a data region in the frequency domain. A PUCCH carrying UCI isallocated to the control region and a PUSCH carrying user data isallocated to the data region. To maintain a single carrier property, aUE does not transmit a PUCCH and a PUSCH simultaneously. A pair of RBsin a subframe is allocated to a PUCCH for a UE. The RBs of the RB pairoccupy different subcarriers in two slots. Thus it is said that the RBpair frequency-hops over a slot boundary.

FIG. 5 illustrates a structure of a DL subframe that may be used inembodiments of the present disclosure.

Referring to FIG. 5, up to three OFDM symbols of a DL subframe, startingfrom OFDM symbol 0 are used as a control region to which controlchannels are allocated and the other OFDM symbols of the DL subframe areused as a data region to which a PDSCH is allocated. DL control channelsdefined for the 3GPP LTE system include a Physical Control FormatIndicator Channel (PCFICH), a PDCCH, and a Physical Hybrid ARQ IndicatorChannel (PHICH).

The PCFICH is transmitted in the first OFDM symbol of a subframe,carrying information about the number of OFDM symbols used fortransmission of control channels (i.e. the size of the control region)in the subframe. The PHICH is a response channel to a UL transmission,delivering an HARQ ACK/NACK signal. Control information carried on thePDCCH is called Downlink Control Information (DCI). The DCI transportsUL resource assignment information, DL resource assignment information,or UL Transmission (Tx) power control commands for a UE group.

2. LTE-U System

2.1 LTE-U System Configuration

Hereinafter, methods for transmitting and receiving data in a CAenvironment of an LTE-A band corresponding to a licensed band and anunlicensed band will be described. In the embodiments of the presentdisclosure, an LTE-U system means an LTE system that supports such a CAstatus of a licensed band and an unlicensed band. A WiFi band orBluetooth (BT) band may be used as the unlicensed band. LTE-A systemoperating on an unlicensed band is referred to as LAA (Licensed AssistedAccess) and the LAA may correspond to a scheme of performing datatransmission/reception in an unlicensed band using a combination with alicensed band.

FIG. 6 illustrates an example of a CA environment supported in an LTE-Usystem.

Hereinafter, for convenience of description, it is assumed that a UE isconfigured to perform wireless communication in each of a licensed bandand an unlicensed band by using two CCs. The methods which will bedescribed hereinafter may be applied to even a case where three or moreCCs are configured for a UE.

In the embodiments of the present disclosure, it is assumed that acarrier of the licensed band may be a primary CC (PCC or PCell), and acarrier of the unlicensed band may be a secondary CC (SCC or SCell).However, the embodiments of the present disclosure may be applied toeven a case where a plurality of licensed bands and a plurality ofunlicensed bands are used in a carrier aggregation method. Also, themethods suggested in the present disclosure may be applied to even a3GPP LTE system and another system.

In FIG. 6, one eNB supports both a licensed band and an unlicensed band.That is, the UE may transmit and receive control information and datathrough the PCC which is a licensed band, and may also transmit andreceive control information and data through the SCC which is anunlicensed band. However, the status shown in FIG. 6 is only example,and the embodiments of the present disclosure may be applied to even aCA environment that one UE accesses a plurality of eNBs.

For example, the UE may configure a macro eNB (M-eNB) and a PCell, andmay configure a small eNB (S-eNB) and an SCell. At this time, the macroeNB and the small eNB may be connected with each other through abackhaul network.

In the embodiments of the present disclosure, the unlicensed band may beoperated in a contention-based random access method. At this time, theeNB that supports the unlicensed band may perform a Carrier Sensing (CS)procedure prior to data transmission and reception. The CS proceduredetermines whether a corresponding band is reserved by another entity.

For example, the eNB of the SCell checks whether a current channel isbusy or idle. If it is determined that the corresponding band is idlestate, the eNB may transmit a scheduling grant to the UE to allocate aresource through (E)PDCCH of the PCell in case of a cross carrierscheduling mode and through PDCCH of the SCell in case of aself-scheduling mode, and may try data transmission and reception.

At this time, the eNB may configure a TxOP including N consecutivesubframes. In this case, a value of N and a use of the N subframes maypreviously be notified from the eNB to the UE through higher layersignaling through the PCell or through a physical control channel orphysical data channel.

2.2 Carrier Sensing (CS) Procedure

In embodiments of the present disclosure, a CS procedure may be called aClear Channel Assessment (CCA) procedure. In the CCA procedure, it maybe determined whether a channel is busy or idle based on a predeterminedCCA threshold or a CCA threshold configured by higher-layer signaling.For example, if energy higher than the CCA threshold is detected in anunlicensed band, SCell, it may be determined that the channel is busy oridle. If the channel is determined to be idle, an eNB may start signaltransmission in the SCell. This procedure may be referred to as LBT.

FIG. 7 is a view illustrating an exemplary Frame Based Equipment (FBE)operation as one of LBT operations.

The European Telecommunication Standards Institute (ETSI) regulation (EN301 893 V1.7.1) defines two LBT operations, Frame Based Equipment (FBE)and Load Based Equipment (LBE). In FBE, one fixed frame is comprised ofa channel occupancy time (e.g., 1 to 10 ms) being a time period duringwhich a communication node succeeding in channel access may continuetransmission, and an idle period being at least 5% of the channeloccupancy time, and CCA is defined as an operation for monitoring achannel during a CCA slot (at least 20 μs) at the end of the idleperiod.

A communication node periodically performs CCA on a per-fixed framebasis. If the channel is unoccupied, the communication node transmitsdata during the channel occupancy time. On the contrary, if the channelis occupied, the communication node defers the transmission and waitsuntil the CCA slot of the next period.

FIG. 8 is a block diagram illustrating the FBE operation.

Referring to FIG. 8, a communication node (i.e., eNB) managing an SCellperforms CCA during a CCA slot [S810]. If the channel is idle [S820],the communication node performs data transmission (Tx) [S830]. If thechannel is busy, the communication node waits for a time periodcalculated by subtracting the CCA slot from a fixed frame period, andthen resumes CCA [S840].

The communication node transmits data during the channel occupancy time[S850]. Upon completion of the data transmission, the communication nodewaits for a time period calculated by subtracting the CCA slot from theidle period [S860], and then resumes CCA [S810]. If the channel is idlebut the communication node has no transmission data, the communicationnode waits for the time period calculated by subtracting the CCA slotfrom the fixed frame period [S840], and then resumes CCA [S810].

FIG. 9 is a view illustrating an exemplary LBE operation as one of theLBT operations.

Referring to FIG. 9(a), in LBE, the communication node first sets q (q{4, 5, . . . , 32}) and then performs CCA during one CCA slot.

FIG. 9(b) is a block diagram illustrating the LBE operation. The LBEoperation will be described with reference to FIG. 9(b).

The communication node may perform CCA during a CCA slot [S910]. If thechannel is unoccupied in a first CCA slot [S920], the communication nodemay transmit data by securing a time period of up to (13/32)q ms [S930].

On the contrary, if the channel is occupied in the first CCA slot, thecommunication node selects N (N∈{1, 2, . . . , q}) arbitrarily (i.e.,randomly) and stores the selected N value as an initial count. Then, thecommunication node senses a channel state on a CCA slot basis. Each timethe channel is unoccupied in one specific CCA slot, the communicationnode decrements the count by 1. If the count is 0, the communicationnode may transmit data by securing a time period of up to (13/32)q ms[S940].

2.3 Discontinuous Transmission in DL

When discontinuous transmission is performed on an unlicensed carrierhaving a limited maximum transmission period, the discontinuoustransmission may influence on several functions necessary for performingan operation of LTE system. The several functions can be supported byone or more signals transmitted at a starting part of discontinuous LAADL transmission. The functions supported by the signals include such afunction as AGC configuration, channel reservation, and the like.

When a signal is transmitted by an LAA node, channel reservation has ameaning of transmitting signals via channels, which are occupied totransmit a signal to other nodes, after channel access is performed viaa successful LBT operation.

The functions, which are supported by one or more signals necessary forperforming an LAA operation including discontinuous DL transmission,include a function for detecting LAA DL transmission transmitted by a UEand a function for synchronizing frequency and time. In this case, therequirement of the functions does not mean that other availablefunctions are excluded. The functions can be supported by other methods.

2.3.1 Time and Frequency Synchronization

A design target recommended by LAA system is to support a UE to make theUE obtain time and frequency synchronization via a discovery signal formeasuring RRM (radio resource management) and each of reference signalsincluded in DL transmission bursts, or a combination thereof. Thediscovery signal for measuring RRM transmitted from a serving cell canbe used for obtaining coarse time or frequency synchronization.

2.3.2 DL Transmission Timing

When a DL LAA is designed, it may follow a CA timing relation betweenserving cells combined by CA, which is defined in LTE-A system (Rel-12or earlier), for subframe boundary adjustment. Yet, it does not meanthat a base station starts DL transmission only at a subframe boundary.Although all OFDM symbols are unavailable in a subframe, LAA system cansupport PDSCH transmission according to a result of an LBT operation. Inthis case, it is required to support transmission of control informationnecessary for performing the PDSCH transmission.

2.4 Measuring and Reporting RRM

LTE-A system can transmit a discovery signal at a start point forsupporting RRM functions including a function for detecting a cell. Inthis case, the discovery signal can be referred to as a discoveryreference signal (DRS). In order to support the RRM functions for LAA,the discovery signal of the LTE-A system and transmission/receptionfunctions of the discovery signal can be applied in a manner of beingchanged.

2.4.1 Discovery Reference Signal (DRS)

A DRS of LTE-A system is designed to support on/off operations of asmall cell. In this case, off small cells correspond to a state thatmost of functions are turned off except a periodic transmission of aDRS. DRSs are transmitted at a DRS transmission occasion with a periodof 40, 80, or 160 ms. A DMTC (discovery measurement timingconfiguration) corresponds to a time period capable of anticipating aDRS received by a UE. The DRS transmission occasion may occur at anypoint in the DMTC. A UE can anticipate that a DRS is continuouslytransmitted from a cell allocated to the UE with a correspondinginterval.

If a DRS of LTE-A system is used in LAA system, it may bring newconstraints. For example, although transmission of a DRS such as a veryshort control transmission without LBT can be permitted in severalregions, a short control transmission without LBT is not permitted inother several regions. Hence, a DRS transmission in the LAA system maybecome a target of LBT.

When a DRS is transmitted, if LBT is applied to the DRS, similar to aDRS transmitted in LTE-A system, the DRS may not be transmitted by aperiodic scheme. In particular, it may consider two schemes described inthe following to transmit a DRS in the LAA system.

As a first scheme, a DRS is transmitted at a fixed position only in aDMTC configured on the basis of a condition of LBT.

As a second scheme, a DRS transmission is permitted at one or moredifferent time positions in a DMTC configured on the basis of acondition of LBT.

As a different aspect of the second scheme, the number of time positionscan be restricted to one time position in a subframe. If it is moreprofitable, DRS transmission can be permitted at the outside of aconfigured DMTC as well as DRS transmission performed in the DMTC.

FIG. 10 is a diagram for explaining DRS transmission methods supportedby LAA system.

Referring to FIG. 10, the upper part of FIG. 10 shows the aforementionedfirst scheme for transmitting a DRS and the bottom part of FIG. 10 showsthe aforementioned second scheme for transmitting a DRS. In particular,in case of the first scheme, a UE can receive a DRS at a positiondetermined in a DMTC period only. On the contrary, in case of the secondscheme, a UE can receive a DRS at a random position in a DMTC period.

In LTE-A system, when a UE performs RRM measurement based on DRStransmission, the UE can perform single RRM measurement based on aplurality of DRS occasions. In case of using a DRS in LAA system, due tothe constraint of LBT, it is difficult to guarantee that the DRS istransmitted at a specific position. Even though a DRS is not actuallytransmitted from a base station, if a UE assumes that the DRS exists,quality of an RRM measurement result reported by the UE can bedeteriorated. Hence, when LAA DRS is designed, it is necessary to permitthe existence of a DRS to be detected in a single DRS occasion. By doingso, it may be able to make the UE combine the existence of the DRS withRRM measurement, which is performed on successfully detected DRSoccasions only.

Signals including a DRS do not guarantee DRS transmissions adjacent intime. In particular, if there is no data transmission in subframesaccompanied with a DRS, there may exist OFDM symbols in which a physicalsignal is not transmitted. While operating in an unlicensed band, othernodes may sense that a corresponding channel is in an idle state duringa silence period between DRS transmissions. In order to avoid theabovementioned problem, it is preferable that transmission burstsincluding a DRS signal are configured by adjacent OFDM symbols in whichseveral signals are transmitted.

2.5 Channel Access Procedure and Contention Window Adjustment Procedure

In the following, the aforementioned channel access procedure and thecontention window adjustment procedure are explained in the aspect of atransmission node.

FIG. 11 is a flowchart for explaining CAP and CWA.

In order for an LTE transmission node (e.g., a base station) to operatein LAA Scell(s) corresponding to an unlicensed band cell for DLtransmission, it may initiate a channel access procedure (CAP) [S1110].

The base station can randomly select a back-off counter N from acontention window (CW). In this case, the N is configured by an initialvalue Ninit [S1120]. The Ninit is randomly selected from among valuesranging from 0 to CW_(p).

Subsequently, if the back-off counter value (N) corresponds to 0[S1122], the base station terminates the CAP and performs Tx bursttransmission including PSCH [S1124]. On the contrary, if the back-offvalue is not 0, the base station reduces the back-off counter value by 1[S1130].

The base station checks whether or not a channel of the LAA Scell(s) isin an idle state [S1140]. If the channel is in the idle state, the basestation checks whether or not the back-off value corresponds to 0[S1150]. The base station repeatedly checks whether or not the channelis in the idle state until the back-off value becomes 0 while reducingthe back-off counter value by 1.

In the step S1140, if the channel is not in the idle state i.e., if thechannel is in a busy state, the base station checks whether or not thechannel is in the idle state during a defer duration (more than 15 usec)longer than a slot duration (e.g., 9 usec) [S1142]. If the channel is inthe idle state during the defer duration, the base station can resumethe CAP [S1144]. For example, when the back-off counter value Ninitcorresponds to 10, if the channel state is determined as busy after theback-off counter value is reduced to 5, the base station senses thechannel during the defer duration and determines whether or not thechannel is in the idle state. In this case, if the channel is in theidle state during the defer duration, the base station performs the CAPagain from the back-off counter value 5 (or, from the back-off countervalue 4 by reducing the value by 1) rather than configures the back-offcounter value Ninit. On the contrary, if the channel is in the busystate during the defer duration, the base station performs the stepS1142 again to check whether or not the channel is in the idle stateduring a new defer duration.

Referring back to FIG. 11, the base station checks whether or not theback-off counter value (N) becomes 0 [S1150]. If the back-off countervalue (N) becomes 0, the base station terminates the CAP and may be ableto transmit a Tx burst including PDSCH.

The base station can receive HARQ-ACK information from a UE in responseto the Tx burst [S1170]. The base station can adjust a CWS (contentionwindow size) based on the HARQ-ACK information received from the UE[S1180].

In the step S1180, as a method of adjusting the CWS, the base stationcan adjust the CWS based on HARQ-ACK information on a first subframe ofa most recently transmitted Tx burst (i.e., a start subframe of the Txburst).

In this case, the base station can set an initial CW to each priorityclass before the CWP is performed. Subsequently, if a probability thatHARQ-ACK values corresponding to PDSCH transmitted in a referencesubframe are determined as NACK is equal to or greater than 80%, thebase station increases CW values set to each priority class to a nexthigher priority.

In the step S1160, PDSCH can be assigned by a self-carrier schedulingscheme or a cross-carrier scheduling scheme. If the PDSCH is assigned bythe self-carrier scheduling scheme, the base station counts DTX,NACK/DTX, or ANY state among the HARQ-ACK information fed back by the UEas NACK. If the PDSCH is assigned by the cross-carrier schedulingscheme, the base station counts the NACK/DTX and the ANY states as NACKand does not count the DTX state as NACK among the HARQ-ACK informationfed back by the UE.

If bundling is performed over M (M>=2) number of subframes and bundledHARQ-ACK information is received, the base station may consider thebundled HARQ-ACK information as M number of HARQ-ACK responses. In thiscase, it is preferable that a reference subframe is included in the Mnumber of bundled subframes.

3. Proposed Embodiment

When a base station or a UE performs LBT (Listen-Before-Talk)-basedsignal transmission in a wireless communication system consisting of thebase station and the UE, a method of transmitting and receiving a signaland a configuration therefor are explained in detail in the presentinvention.

According to the present invention, a base station or a UE shouldperform LBT to transmit a signal on an unlicensed band. When the basestation or the UE transmits a signal, it is necessary to make signalinterference not to be occurred with different communication nodes suchas Wi-Fi, and the like. For example, according to Wi-Fi standard, a CCAthreshold value is regulated by −62 dBm and −82 dBm for a non-Wi-Fisignal and a Wi-Fi signal, respectively. In particular, if an STA(station) or an AP (access point) senses a signal received with power(or energy) equal to or greater than −62 dBm rather than Wi-Fi, the STAor the AP does not perform signal transmission.

In this case, it may be difficult to always guarantee DL transmission ofan eNB or UL transmission of a UE on an unlicensed. Hence, a UEoperating on the unlicensed band may maintain access with a differentcell operating on a licensed band to stably control mobility, RRM (radioresource management) function, and the like. In the present invention,for clarity, a cell accessed by a UE on the unlicensed band is referredto as a U-Scell (or LAA Scell) and a cell accessed by the UE on thelicensed band is referred to as a Pcell. As mentioned in the foregoingdescription, a scheme of performing data transmission/reception on theunlicensed band using a combination with the licensed band is generallycalled LAA (licensed assisted access).

TABLE 2 Channel Access Priority allowed Class (p) m_(p) CW_(min, p)CW_(max, p) T_(mcot, p) CW_(p) sizes 1 1 3 7    2 ms {3, 7} 2 1 7 15   3 ms  {7, 15} 3 3 15 63 8 or 10 ms {15, 1, 63} 4 7 15 1023 8 or 10 ms{15, 31, 63, 127, 255, 511, 1023}

As shown in Table 2, in Rel-13 LAA system, 4 channel access priorityclasses are defined in total. And, a length of a defer period, a CWS,MCOT (maximum channel occupancy time), and the like are definedaccording to each of the channel access priority classes. Hence, when aneNB transmits a downlink signal via an unlicensed band, the eNB performsrandom backoff by utilizing LBT parameters determined according to achannel access priority class and may be then able to access a channelduring limited maximum transmission time only after the random backoffis completed.

For example, in case of the channel access priority class 1/2/3/4, themaximum channel occupancy time (MCOT) is determined by 2/3/8/8 ms. Themaximum channel occupancy time (MCOT) is determined by 2/3/10/10 ms inenvironment where other RAT such as Wi-Fi does not exists (e.g., bylevel of regulation).

As shown in Table 2, a set of CWSs capable of being configured accordingto a class is defined. One of points different from Wi-Fi system is inthat a different backoff counter value is not defined according to achannel access priority class and LBT is performed using a singlebackoff counter value (this is referred to as single engine LBT).

For example, when an eNB intends to access a channel via an LBToperation of class 3, since CWmin (=15) is configured as an initial CWS,the eNB performs random backoff by randomly selecting an integer fromamong numbers ranging from 0 to 15. If a backoff counter value becomes0, the eNB starts DL Tx and randomly selects a new backoff counter for anext Tx burst after the DL Tx burst is completed. In this case, if anevent for increasing a CWS is triggered, the eNB increases a size of theCWS to 31 corresponding to a next size, randomly selects an integer fromamong numbers ranging from 0 to 31, and performs random backoff.

In this case, when a CWS of the class 3 is increased, CWSs of allclasses are increased as well. In particular, if the CW of the class 3becomes 31, a CWS of a class 1/2/4 becomes 7/15/31. If an event fordecreasing a CWS is triggered, CWS values of all classes are initializedby CWmin irrespective of a CWS value of the triggering timing.

FIG. 12 is a diagram illustrating a partial TTI or a partial subframeapplicable to the present invention.

In Rel-13 LAA system, MCOT is utilized as much as possible when DL Txburst is transmitted. In order to support consecutive transmission, apartial TTI, which is defined as DwPTS, is introduced. The partial TTI(or partial subframe) corresponds to a section in which a signal istransmitted as much as a length shorter than a legacy TTI (e.g., 1 ms)when PDSCH is transmitted.

In the present invention, for clarity, a starting partial TTI or astarting partial subframe corresponds to a form that a part of symbolspositioned at the fore part of a subframe are emptied out. An endingpartial TTI or an ending partial subframe corresponds to a form that apart of symbols positioned at the rear part of a subframe are emptiedout. (On the contrary, an intact TTI is referred to as a normal TTI or afull TTI.)

FIG. 12 illustrates various types of the aforementioned partial TTI. Thefirst drawing of FIG. 12 illustrates an ending partial TTI (or subframe)and the second drawing illustrates a starting partial TTI (or subframe).The third drawing of FIG. 12 illustrates a partial TTI (or subframe)that a part of symbols positioned at the fore part and the rear part ofa subframe are emptied out. In this case, when signal transmission isexcluded from a normal TTI, a time section during which the signaltransmission is excluded is referred to as a transmission gap (TX gap).

Although the present invention is explained on the basis of a DLoperation in FIG. 12, the present invention can also be identicallyapplied to a UL operation. For example, a partial TTI structure shown inFIG. 12 can be applied to a form of transmitting PUCCH or PUSCH as well.

Based on the abovementioned technical contents, the present inventionproposes a method of transmitting and receiving a signal (DwPTS, SRS(Sounding Reference Signal), etc.) in a situation that carriersincluding carriers of an unlicensed band are aggregated.

3.1. DwPTS

In an LAA Scell to which the present invention is applicable, an endingpartial subframe illustrated in FIG. 12 has been introduced due to thereason described in the following.

-   -   Maximum channel occupancy time (MCOT) is defined for coexistence        with other nodes on an unlicensed band. (When transmission        starting time is not matched with a subframe boundary), in order        to utilize the MCOT as much as possible, it is necessary for an        eNB to configure the last subframe of contiguous downlink burst        (hereinafter, DL burst) using a partial subframe shorter than 1        ms.    -   When the last subframe of the contiguous DL burst is configured        by a full subframe of 1 ms, if LBT for transmitting a next        contiguous DL burst is considered, at least 1 subframe is wasted        without transmitting any data. On the contrary, if the last        subframe of DL burst is configured by a partial subframe shorter        than 1 ms and an eNB completes LBT during a timing gap between        subframe boundaries, DL burst transmission is enabled from a        subsequent subframe.

In this case, as illustrated in Table 3, basically, an ending partialsubframe applicable to the present invention may have a structureidentical to a DwPTS of a frame structure 2 (TDD).

TABLE 3 Normal cyclic prefix in downlink Extended cyclic prefix indownlink Special UpPTS UpPTS subframe Normal cyclic Extended cyclicNormal cyclic Extended cyclic configuration DwPTS prefix in uplinkprefix in uplink DwPTS prefix in uplink prefix in uplink 0  6592 · T_(s)(1 + X) · 2192 · T_(s) (1 + X) · 2560 · T_(s)  7680 · T_(s) (1 + X) ·2192 · T_(s) (1 + X) · 2560 · T_(s) 1 19760 · T_(s) 20480 · T_(s) 221952 · T_(s) 23040 · T_(s) 3 24144 · T_(s) 25600 · T_(s) 4 26336 ·T_(s)  7680 · T_(s) (2 + X) · 2192 · T_(s) (2 + X) · 2560 · T_(s) 5 6592 · T_(s) (2 + X) · 2192 · T_(s) (2 + X) · 2560 · T_(s) 20480 ·T_(s) 6 19760 · T_(s) 23040 · T_(s) 7 21952 · T_(s) 12800 · T_(s) 824144 · T_(s) — — — 9 13168 · T_(s) — — —

In this case, if the number of PDCCH symbols of an ending partialsubframe also follows a structure identical to DwPTS, (identical to“Subframe 1 and 6 for frame structure type 2” of Table 4) the number ofPDCCH symbols of the ending partial subframe can be configured bymaximum 2 symbols.

TABLE 4 Number of Number of OFDM symbols OFDM symbols for PDCCH when forPDCCH when Subframe N_(RB) ^(DL) > 10 N_(RB) ^(DL) < 10 Subframe 1 and 6for frame 1, 2 2 structure type 2 MBSFN subframes on a carrier 1, 2 2supporting PDSCH, configured with 1 or 2 cell-specific antenna portsMBSFN subframes on a carrier 2 2 supporting PDSCH, configured with 4cell-specific antenna ports Subframes on a carrier 0 0 not supportingPDSCH Non-MBSFN subframes (except 1, 2, 3 2, 3 subframe 6 for framestructure type 2) configured with positioning reference signals Allother cases 1, 2, 3 2, 3, 4

According to a frame structure 2 of a legacy LTE system, similar to aspecial subframe configuration 0/5, if a DwPTS is configured by 3symbols, PDSCH is not transmitted during the DwPTS, PDCCH/CRS(Cell-specific Reference Signal) is transmitted during maximum 2symbols, and a PSS (Primary Synchronization Signal) is transmitted atthe last 3^(rd) symbol.

On the contrary, in an LAA Scell to which the present invention isapplicable (i.e., frame structure 3), a PSS can be transmitted at thelast symbol of the first slot of an SF #0/5 (or a DRS (DiscoveryReference Signal) SF). In other word, if a DwPTS having a length of 3symbols is transmitted in the LAA Scell to which the present inventionis applicable, a DL signal or a channel expected by a UE may not existin the last 3^(rd) symbol. Hence, the present invention proposes amethod of efficiently utilizing the remaining resource in the following.

3.1.1. First Method of Configuring DwPTS

The present invention proposes a method of allowing maximum 3 symbols ofa PDCCH region to a DwPTS having a length of 3 symbols. In other word,when a DwPTS having a length of 3 symbols is transmitted in an LAA Scellto which the present invention is applicable, the number of OFDM symbolsconstructing a PDCCH may correspond to minimum 1 to maximum 3.

When an eNB transmits a UL grant via a partial subframe, in particular,when multi-SF scheduling for UL is introduced, a size of a correspondingscheduling grant may increase. In this case, as a PDCCH region isgetting bigger, it is more profitable in the aspect of multi-SFscheduling transmission flexibility. In particular, in case ofconsidering multi-SF UL scheduling, it may be able to allow a PDCCHregion to have a symbol length as many as maximum 3 symbols not only fora DwPTS having a length of 3 symbols but also for a general DwPTS.

In this case, the number of REs available for PDSCH in a DwPTS can bedecreased due to the increased number of PDCCH symbols, therebyconsiderably decreasing a coding rate. In order to prevent the decreaseof the coding rate, it may be able to introduce a scaling factor smallerthan a legacy scaling factor to a DwPTS. For example, a scaling factorvalue 0.6 is applied for special subframe configurations 1/2/3/4/6/7/8and a scaling factor value 0.3 can be applied for a special subframeconfiguration 9. Or, it may be able to introduce a scaling factorsmaller than a legacy scaling factor only when a length of PDCCH in aDwPTS corresponds to 3 symbols.

3.1.2. Second Method of Configuring DwPTS

The present invention proposes a method of correcting a length using aDwPTS of a length of 2 symbols instead of a DwPTS of a length of 3symbols. In particular, it may be able to introduce an ending partialsubframe having a length of 4400 T_s instead of a special subframeconfiguration 0/5.

In an LAA Scell to which the present invention is applicable, a commonPDCCH has been introduced to indicate an OFDM symbol length of an endingpartial subframe in a subframe immediately before the ending partialsubframe and the ending partial subframe. In this case, ‘Subframeconfiguration for LAA’ field constructing the common PDCCH is shown inTable 5 in the following.

In this case, according to the present invention, a configurationcorresponding to ‘0110’ can be changed with (−,2) and a configurationcorresponding to ‘1101’ can be changed with (2,−). By doing so, it isable to minimize dummy signal transmission of an eNB.

TABLE 5 Value of Configuration of occupied ‘Subframe configuration forOFDM symbols LAA’ field in current (current subframe, next subframesubframe) 0000 (—, 14) 0001 (—, 12) 0010 (—, 11) 0011 (—, 10) 0100 (—,9)  0101 (—, 6)  0110 (—, 3)  0111 (14, *)  1000 (12, —) 1001 (11, —)1010 (10, —) 1011  (9, —) 1100  (6, —) 1101  (3, —) 1110 reserved 1111reserved NOTE: (—, Y) means UE may assume the first Y symbols areoccupied in next subframe and other symbols in the next subframe are notoccupied. (X, —) means UE may assume the first X symbols are occupied incurrent subframe and other symbols in the current subframe are notoccupied. (X, *) means UE may assume the first X symbols are occupied incurrent subframe, and at least the first OFDM symbol of the nextsubframe is not occupied.

3.2. SRS without PUSCH

3.2.1. When SRS is Transmitted without PUSCH, LBT Method of UE

In a release-14 eLAA (enhanced LAA) system to which the presentinvention is applicable, similar to a legacy LTE system, a UE cantransmit an SRS at the last symbol of a specific subframe. In this case,the specific subframe may correspond to an ending partial subframe or anormal UL subframe. In particular, the UE can transmit an SRS in thelast symbol of an ending partial subframe or can transmit an SRS in thelast symbol of a normal UL subframe.

In this case, when the UE transmits the SRS, it may be able to applyvarious LBTs to transmit the SRS. For example, it may allow the UE totransmit the SRS without LBT (LBT type 1), it may allow the UE totransmit the SRS when a channel is idle after X usec (e.g., X=25)CCA-based LBT (LBT type 2), or it may allow the UE to transmit the SRSwhen LBT accompanied with random backoff is completed (LBT type 3).

In this case, SRS triggering can be performed by at least one selectedfrom the group consisting of a DL grant, a UL grant, and a common PDCCH.When a UE performs LBT to transmit an SRS, the LBT can also be indicatedby a DL grant, a UL grant, a common PDCCH, and the like. In this case,when information on LBT to be performed and information on LBT to beperformed by a UE is signaled by a common PDCCH, the LBT can be commonlyapplied not only to an SRS but also to a different UL channel (PRACH,PUSCH, PUCCH, etc.).

In the following, a method of performing LBT performed by a UE totransmit an SRS is explained in detail according to the presentinvention.

3.2.1.1. First LBT Method

When transmission of an SRS is triggered in the last symbol of an endingpartial subframe, the present invention proposes a method of determiningan LBT type according to a length of the ending partial subframe. Inother word, when a length of an ending partial subframe obtained by acommon PDCCH is less than Y, an LBT type performed by a UE can beconfigured by an LBT type A. When the length of the ending partialsubframe is equal to or greater than Y, the LBT can be configured by anLBT type B. For example, in order to transmit an SRS after a DwPTS of alength of 12 symbols, the UE may attempt to perform an LBT type 1. Inorder to transmit an SRS after a DwPTS of a different length, the UE mayattempt to perform an LBT type 2. Or, it may be able to configure the UEto perform the abovementioned LBT.

3.2.1.2. Second LBT Method

When LBT type 3 is applied as LBT for transmitting an SRS, the presentinvention proposes a method of applying an LBT parameter correspondingto a specific priority class among channel access priority classes setto the LBT type 3 in advance. In other word, when an LBT type performedby a UE to transmit an SRS is configured by LBT type 3 (randombackoff-based LBT), the UE attempts to perform LBT to which an LBTparameter corresponding to a priority class selected from among thepredetermined channel access priority classes and can perform SRStransmission based on a result of the LBT.

In this case, it is able to apply a channel access priority class for DLLBT defined in LTE release-13 or a channel access priority classseparately configured for UL LBT.

In this case, the LBT type 3 can be indicated by a DL grant, a UL grant,a common PDCCH, or the like. If no LBT type is indicated by the DLgrant, the UL grant, the common PDCCH, or the like, a default LBT typecan be configured.

In this case, it is able to configure a specific CWS value only amongCWS set values of a specific priority class as an LBT parameter for theLBT. For example, when a priority class 1 is configured by RRC signalingas an LBT parameter for the LBT, an applicable CWS set may correspond to{3,7}. In particular, 3 or 7 corresponding to one of values of the CWSset values can be applied as a CWS value applied to the LBT to transmitan SRS. As a different example, priority class 3 is configured by RRCsignaling as an LBT parameter for the LBT and {15,31} among {15,31,63}can be applied only as a CWS set applied to the LBT. The CWS set can beconfigured by RRC signaling.

As an additional example, if multiple CWSs are configured for SRStransmission, it may be able to define a rule that a CWS is increasedwhenever SRS transmission fails and a CWS is reset whenever SRStransmission succeeds irrespective of whether or not a CWS is adjustedfor PUSCH.

As a different additional example, it may be able to define a rule thata UE performs LBT by utilizing a CWS corresponding to a most recentlyused CWS to transmit PUSCH. Specifically, when a CWS value of LBT fortransmitting an SRS is configured based on a priority class 2, in orderto transmit an SRS, a UE can perform LBT based on a CWS identical to apriority class 2-related CWS most recently used for transmitting PUSCH.As a different example, a CWS value of LBT for transmitting an SRS canbe determined in consideration of whether the CWS value applied to theLBT corresponds to a configured CWmin or an increased CWS onlyirrespective of a priority class of LBT most recently applied by a UE totransmit PUSCH. Specifically, when a CWS value of LBT for transmittingan SRS is configured based on a priority class 2, although a CWS of LBTmost recently applied by a UE to transmit PUSCH corresponds to a CWS ofa priority class 3, a CWS value of LBT for transmitting an SRS can beconfigured based on a minimum value or an increased CWS among configuredvalues according to whether the CWS value corresponds to CWmin or anincreased CWS. As a further different example, a UE can perform LBT byidentically utilizing a CWS most recently applied to LBT fortransmitting PUSCH irrespective of a priority class of LBT fortransmitting an SRS.

3.2.1.3. Third LBT Method

The present invention proposes a method of not allowing a reservationsignal for transmitting an SRS without PUSCH. Since SRS transmission isperformed during a symbol length, transmission of a reservation signalfor the SRS transmission may become overhead. Specifically, when a UEperforms LBT of an LBT type 2 to transmit an SRS, the UE can perform theLBT during X usec (e.g., X=25) immediately before the 14^(th) symbolboundary. Or, when the UE performs LBT of an LBT type 3 to transmit anSRS, the UE can match LBT ending timing with timing immediately beforethe 14^(th) symbol boundary via self-deferral. In this case, whether ornot a reservation signal for SRS transmission without PUSCH is allowedcan be configured via RRC signaling.

The abovementioned first to third LBT methods can be utilized not onlyfor SRS transmission but also for PUSCH/PUCCH/PRACH transmission afterDwPTS (i.e., UpPTS position). Specifically, the PUSCH/PUCCH/PRACH can beconfigured by a partial subframe only shorter than 1 ms between DwPTS+GP(guard period) and a next subframe boundary.

3.2.2. Method of Transmitting SRS without PUSCH

In the present chapter, a method of configuring a gap between an SRSscheduled to be transmitted without PUSCH in the same subframe and PUSCHscheduled in a next subframe is proposed.

FIG. 13 is a diagram for explaining an operation of a UE when the UE isscheduled to transmit PUSCH in an SF #n+1 after the UE transmits an SRSin the last symbol of an SF #n according to one embodiment of thepresent invention.

For example, as shown in FIG. 13, when a UE is scheduled to transmitPUSCH in SF #n+1 after an SRS is transmitted in the last symbol of SF#n, a starting position of the PUSCH can be indicated by a positionappearing after 25 usec or 25 usec+TA (Timing Advance) from a boundaryof the SF #n+1 in the SF #n+1. In this case, it is necessary to considerwhether or not a UE is able to perform all operations including anoperation of switching into a reception operation (e.g., TX→RXswitching) and LBT (e.g., 25 usec LBT) (in addition, switching into atransmission operation (e.g., RX→TX switching)) during a short gap asmuch as 25 usec or 25 usec+TA.

In this case, the switching into the transmission operation (e.g., RX→TXswitching) can be included in PUSCH transmission time in the SF #n+1. Inthe following, the present invention proposes a method for a UE totransmit an SRS in the SF #n and transmit PUSCH in the SF #n+1 in theabovementioned situation.

3.2.2.1. First Method of Transmitting SRS and PUSCH

When an eNB schedules a UE to transmit SF #n SRS and SF #n+1 PUSCH, thepresent invention proposes a method of setting a limit on a gap sizebetween two subframes with a value of 25 usec (+TA). In this case, whenthe UE is scheduled to transmit the SF #n SRS and the SF #n+1 PUSCH, theUE may expect DFTS-OFDM symbol #0 or DFTS-OFDM symbol #1 only assignaling for a PUSCH starting position of the SF #n+1.

3.2.2.2 Second Method of Transmitting SRS and PUSCH

Although a PUSCH starting position is indicated by a position appearingafter 25 usec or 25 usec+TA from SF #n+1 boundary, it may be difficultfor a scheduled UE to perform all operations including switching to areception operation (TX→RX switching)+25 usec LBT (+ switching into atransmission operation (RX→TX switching)) during a gap between anindicated SRS and PUSCH. In this case, the present invention proposes amethod for a UE to start PUSCH transmission from the timing at which areception operation (TX→RX switching)+25 usec LBT (+ switching into atransmission operation (RX→TX switching)) are completed.

If a UE completes a series of operations including switching to areception operation (TX→RX switching)+25 usec LBT (+ switching into atransmission operation (RX→TX switching)) prior to a DFTS-OFDM symbol #1(i.e., second SC-FDM symbol), the UE can transmit an initial signal(e.g., a predetermined signal such as DM-RS/SRS, a part (extension of aCP (cyclic prefix)) of PUSCH to be transmitted in the DFTS-OFDM symbol#1 from the timing. Or, if it is difficult for a scheduled UE to performall operations including switching to a reception operation (TX→RXswitching)+25 usec LBT (+ switching into a transmission operation (RX→TXswitching)) during a gap between an indicated SRS and PUSCH, it is ableto define a rule that the UE attempts to transmit PUSCH from theDFTS-OFDM symbol #1 boundary.

3.2.2.3. Third Method of Transmitting SRS and PUSCH

When time taken for a UE to perform all operations including switchingto a reception operation (TX→RX switching)+25 usec LBT (+ switching intoa transmission operation (RX→TX switching)) corresponds to X (e.g., 45)us, the present invention proposes a method of indicating a PUSCHstarting position by a position appearing after X us. In this case, avalue of the X is configured via higher layer signaling or can bedifferently configured according to capability signaling reported by aUE.

Specifically, if a PUSCH starting position is indicated by a positionappearing after X usec from SF #n+1 boundary, a UE can transmit PUSCH inthe SF #n+1 after switching to a reception operation (TX→RXswitching)+25 usec LBT (+ switching into a transmission operation (RX→TXswitching)) are performed. In this case, signaling indicating that thePUSCH starting position corresponds to a position appearing after the Xus from a subframe boundary can be additionally introduced to signalingindicating a legacy PUSCH starting position (e.g., DFTS-OFDM symbol #0,DFTS-OFDM symbol #0+25 usec, DFTS-OFDM symbol #0+25 usec+TA, orDFTS-OFDM symbol #1). Or, when the PUSCH starting position is indicatedby DFTS-OFDM symbol #0+25 usec (and/or DFTS-OFDM symbol #0+25 usec+TA),the PUSCH starting position can be configured to be interpreted asDFTS-OFDM symbol #0+X usec only when an SRS is scheduled in an SF #n anda PUSCH is scheduled in an SF #n+1.

3.2.2.4. Fourth Method of Transmitting SRS and PUSCH

The present invention proposes a method of making an on/off powertransient period to be absorbed in an SRS symbol by changing a powermask of an SRS. In other word, the present invention proposes a methodof including the on/off power transient period in an SRS symbol period.

In particular, among operations including switching to a receptionoperation (TX→RX switching)+25 usec LBT (+ switching into a transmissionoperation (RX→TX switching)), time for switching to the transmissionoperation is absorbed (or included) in SF #n+1 PUSCH period and time forswitching to the reception operation can be absorbed (or included) in SF#n SRS symbol period. In particular, although a PUSCH starting positionis indicated by a position appearing after 25 usec or 25 usec+TA from SF#n+1 boundary, a UE according to the present invention can easilyimplement a corresponding operation.

3.2.2.5. Fifth Method of Transmitting SRS and PUSCH

The present invention propose a method of allowing a UE to transmit aninitial signal from the last symbol boundary prior to a subframe inwhich a scheduled PUSCH is transmitted and a method of introducingsignaling for supporting the method. In consideration of motivation ofan eNB that schedules a small gap of 25 used or 25 usec+TA between anSRS and a following PUSCH, it may additionally consider a multiplexingmethod with a different UE transmitting a signal in an SF #n+1 (nottransmitting an SRS in an SF #n). To this end, a UE 1 configured totransmit SF #n SRS+SF #n+1 is scheduled to make no gap between an SRSand a PUSCH. A UE 2 configured to transmit SF #n+1 PUSCH only can beindicated to transmit an initial signal from the last symbol of the SF#n. In this case, as the initial signal, it may apply a predeterminedsignal such as DM-RS/SRS or a part (extension of a CP (cyclic prefix))of PUSCH to be transmitted in DFTS-OFDM symbol #0 of the SF #n.

3.2.2.6. Sixth Method of Transmitting SRS and PUSCH

When a short transmission-reception transition (TX-RX transition) isavailable (e.g., 5 us), the present invention proposes a method ofincluding a power transient period in a small gap of 25 usec or 25usec+TA. For example, it may be able to define that an on/off powertransient period of an SRS is to be absorbed (included) in a part of asmall gap of 25 usec or 25 usec+TA in SF #n and a UE performs CCA in aremaining period only.

The aforementioned first to sixth methods of transmitting an SRS and aPUSCH can be identically applied not only to a case that a UE isscheduled to transmit an SRS in SF #n and transmit PUSCH in SF #n+1 butalso to a case that the UE is scheduled to transmit PUSCH in the SF #nand transmit PUSCH in the SF #n+1. In particular, when a specific UE isscheduled to transmit PUSCH in an SF #n and PUSCH in an SF #n+1 during asmall gap of 25 usec or 25 usec+TA between, the UE can transmit thePUSCH in the SF #n and transmit the PUSCH in the SF #n+1 by applying amethod similar to the first to sixth methods of transmitting SRS andPUSCH. For example, when the fourth method of transmitting an SRS and aPUSCH is performed, among operations including switching to a receptionoperation (TX→RX switching)+25 usec LBT (+ switching into a transmissionoperation (RX→TX switching)), time for switching to the transmissionoperation is absorbed (or included) in SF #n+1 PUSCH duration and timefor switching to the reception operation can be absorbed (or included)in SF #n PUSCH duration.

The abovementioned characteristics are more generally summarized in thefollowing. In the aspect of the same UE, when a small gap of 25 usec or25 usec+TA is scheduled between SRS/PUSCH/PUCCH/PRACH transmission in SF#n and PUSCH/PUCCH/PRACH transmission in SF #n+1, it may apply theaforementioned methods.

In LAA Scell to which the present invention is applicable, it is able toallow a plurality of UL subframes to be scheduled in single DCI. To thisend, unlike a DCI format 0/4 scheduling a legacy UL subframe, it is ableto define a new DCI format 0A/0B/4A/4B.

In this case, the DCI format 0A/0B can be used for scheduling 1 TB(transmission block) and the DCI format 4A/4B can be used for scheduling2 TBs. And, the DCI format 0A/4A can be used for scheduling a single ULsubframe and the DCI format 0B/4B can be used for scheduling maximum 4UL subframes at a time.

In this case, the maximum number of subframes capable of being scheduledby the DCI format 0B/4B can be configured via RRC signaling and thenumber of subframes can be configured by a value selected from among 2to 4. In this case, the number of subframes equal to or less than themaximum value can be actually scheduled according to DCI. The number ofactually scheduled subframes can be dynamically indicated via a ULgrant. And, the DCI format 0B/4B can signal a PUSCH starting positionand can indicate a PUSCH starting gap not only for a first subframe butalso for the remaining subframes among a plurality of UL subframesscheduled at the same time.

A position of a subframe in which an SRS is to be transmitted can beindicated by a legacy “SRS request” bit field and a subframe position tobe transmitted among information corresponding to each state can beconfigured to be indicated via RRC signaling. In this case, amongsubframes scheduled via the DCI format 0B/4B, a PUSCH starting positionof a subframe(s) to which a PUSCH starting gap is indicated can beconfigured to be determined by a different rule according to a subframein which an SRS is transmitted.

Specifically, among subframes scheduled via the DCI format 0B/4B, if astarting gap is indicated to a subframe (e.g., SF #n+1) appearing aftera subframe (e.g., SF #n) in which an SRS is transmitted, it may applythe aforementioned first to sixth methods of transmitting an SRS andPUSCH to transmit an SRS in the SF #n and transmit PUSCH in the SF #n+1.

For example, when the first method of transmitting an SRS and PUSCH isapplied and a DCI format 0B/4B indicates 4 subframes including SF#1/2/3/4 to be scheduled, assume a case that a starting gap is indicatedto all of the 4 subframes. If SRS transmission is indicated in the SF#2, a DFTS-OFDM symbol #0+25 usec or a DFTS-OFDM symbol #0+25 usec+TA isapplied as a starting position of the SF #2/4. In this case, it mayapply a DFTS-OFDM symbol #1 as a starting position of the SF #3.

As a different example, if there is a subframe in which SRS transmissionis indicated (i.e., if SRS transmission is triggered) among subframesscheduled via the DCI format 0B/4B, it may apply the aforementionedfirst to sixth methods of transmitting an SRS and PUSCH to transmit theremaining subframe(s) except the first subframe among the scheduledsubframes. For example, when the first method of transmitting an SRS andPUSCH is applied and a DCI format 0B/4B indicates 4 subframes includingSF #1/2/3/4 to be scheduled, assume a case that a starting gap isindicated to all of the 4 subframes. If SRS transmission is indicated inthe SF #2, it may apply a DFTS-OFDM symbol #1 as a starting position ofthe SF #2/3/4.

Based on the aforementioned contents, the present invention proposes amethod for a UE to transmit an SRS in the following.

Due to the characteristic of an unlicensed band, a UE performs LBT totransmit an SRS. If the LBT succeeds, the UE can transmit the SRS.

In this case, if the SRS transmission does not include PUSCHtransmission, it may apply random backoff-based LBT based on apredetermined channel access priority class.

In this case, the SRS transmission can be performed in the last symbolin time dimension among symbols included in a subframe.

And, when LBT is performed to transmit an SRS without PUSCH, it mayapply random backoff-based LBT based on a channel access priority classhaving a smallest contention widow size among a plurality of channelaccess priority classes configured in a system. In other word, when a UEintends to perform SRS transmission only without transmitting PUSCH, theUE can perform LBT of a highest success rate among random backoff basedLBTs to transmit an SRS.

For example, when LBT is performed to transmit an SRS withouttransmitting PUSCH, it may apply random backoff-based LBT based on achannel access priority class having a value of {3,7} as an allowedcontention window size.

Moreover, a contention window size applied to the LBT for performing SRStransmission without PUSCH can be differently applied according towhether a contention window size recently applied to LBT fortransmitting PUSCH corresponds to a minimum contention window size or anincreased contention window size.

Specifically, if the contention window size recently applied to LBT fortransmitting PUSCH corresponds to a minimum contention window size, itmay apply 3 as a contention window size applied to the LBT fortransmitting an SRS without PUSCH. Or, if the contention window sizerecently applied to LBT for transmitting PUSCH corresponds to anincreased contention window size, it may apply 7 as a contention windowsize applied to the LBT for transmitting an SRS without PUSCH.

Since it is able to include the examples for the proposed method as oneof implementation methods of the present invention, it is apparent thatthe examples are considered as a sort of proposed methods. Although theembodiments of the present invention can be independently implemented,the embodiments can also be implemented in a combined/aggregated form ofa part of embodiments. It may define a rule that an eNB informs a UE ofinformation on whether to apply the proposed methods (or, information onrules of the proposed methods) via a predefined signal (e.g., physicallayer signal or higher layer signal).

4. Device Configuration

FIG. 14 is a diagram illustrating configurations of a UE and a basestation capable of being implemented by the embodiments proposed in thepresent invention. A UE and a base station illustrated in FIG. 14operate to implement embodiments for a method of transmitting andreceiving a sounding reference signal without a physical uplink sharedchannel between the UE and the base station.

A UE 1 may act as a transmission end on a UL and as a reception end on aDL. A base station (eNB) 100 may act as a reception end on a UL and as atransmission end on a DL.

That is, each of the UE and the base station may include a Transmitter(Tx) 10 or 110 and a Receiver (Rx) 20 or 120, for controllingtransmission and reception of information, data, and/or messages, and anantenna 30 or 130 for transmitting and receiving information, data,and/or messages.

Each of the UE and the base station may further include a processor 40or 140 for implementing the afore-described embodiments of the presentdisclosure and a memory 50 or 150 for temporarily or permanently storingoperations of the processor 40 or 140.

The UE performs LBT for transmitting an SRS via the processor 4. If theLBT succeeds, the UE can transmit the SRS. In this case, if the SRStransmission does not include PUSCH transmission, it may apply randombackoff-based LBT based on a predetermined channel access priorityclass.

The Tx and Rx of the UE and the base station may perform a packetmodulation/demodulation function for data transmission, a high-speedpacket channel coding function, OFDM packet scheduling, TDD packetscheduling, and/or channelization. Each of the UE and the base stationof FIG. 14 may further include a low-power Radio Frequency(RF)/Intermediate Frequency (IF) module.

Meanwhile, the UE may be any of a Personal Digital Assistant (PDA), acellular phone, a Personal Communication Service (PCS) phone, a GlobalSystem for Mobile (GSM) phone, a Wideband Code Division Multiple Access(WCDMA) phone, a Mobile Broadband System (MBS) phone, a hand-held PC, alaptop PC, a smart phone, a Multi Mode-Multi Band (MM-MB) terminal, etc.

The smart phone is a terminal taking the advantages of both a mobilephone and a PDA. It incorporates the functions of a PDA, that is,scheduling and data communications such as fax transmission andreception and Internet connection into a mobile phone. The MB-MMterminal refers to a terminal which has a multi-modem chip built thereinand which can operate in any of a mobile Internet system and othermobile communication systems (e.g. CDMA 2000, WCDMA, etc.).

Embodiments of the present disclosure may be achieved by various means,for example, hardware, firmware, software, or a combination thereof.

In a hardware configuration, the methods according to exemplaryembodiments of the present disclosure may be achieved by one or moreApplication Specific Integrated Circuits (ASICs), Digital SignalProcessors (DSPs), Digital Signal Processing Devices (DSPDs),Programmable Logic Devices (PLDs), Field Programmable Gate Arrays(FPGAs), processors, controllers, microcontrollers, microprocessors,etc.

In a firmware or software configuration, the methods according to theembodiments of the present disclosure may be implemented in the form ofa module, a procedure, a function, etc. performing the above-describedfunctions or operations. A software code may be stored in the memory 180or 190 and executed by the processor 120 or 130. The memory is locatedat the interior or exterior of the processor and may transmit andreceive data to and from the processor via various known means.

Those skilled in the art will appreciate that the present disclosure maybe carried out in other specific ways than those set forth hereinwithout departing from the spirit and essential characteristics of thepresent disclosure. The above embodiments are therefore to be construedin all aspects as illustrative and not restrictive. The scope of thedisclosure should be determined by the appended claims and their legalequivalents, not by the above description, and all changes coming withinthe meaning and equivalency range of the appended claims are intended tobe embraced therein. It is obvious to those skilled in the art thatclaims that are not explicitly cited in each other in the appendedclaims may be presented in combination as an embodiment of the presentdisclosure or included as a new claim by a subsequent amendment afterthe application is filed.

INDUSTRIAL APPLICABILITY

The embodiments of the present invention can be applied to variouswireless access systems including 3GPP (3rd Generation PartnershipProject) and 3GPP2 system. The embodiments of the present invention canbe applied not only to various wireless access systems but also to alltechnical fields to which the various wireless access systems areapplied. Further, the proposed method can also be applied to an mmWavecommunication system using ultrahigh frequency band.

What is claimed is:
 1. A method of transmitting a sounding referencesignal (SRS) by a user equipment (UE) to a base station in a wirelesscommunication system supporting an unlicensed band, the methodcomprising: performing a channel access procedure (CAP) for SRStransmission, and transmitting the SRS via the unlicensed band based onthe CAP, wherein the CAP is a random backoff-based CAP and apredetermined channel access priority class has a value of {3,7} as anallowed contention window size, wherein, based on a contention windowsize most recently applied to the CAP for transmitting a physical uplinkshared channel (PUSCH) being a minimum contention window size, the value3 is applied as the contention window size applied to the CAP fortransmitting the SRS, and wherein, based on the contention window sizemost recently applied to the CAP for transmitting the PUSCH being anincreased window size relative to the minimum contention window size,the value 7 is applied as the contention window size applied to the CAPfor transmitting the SRS.
 2. The method of claim 1, the randombackoff-based CAP comprising: randomly selecting a counter N between 0and a contention window size for a SRS transmission, wherein the counterN is decremented by 1 based on determination that the unlicensed band issensed to be idle during a defer duration; and determining that thecounter N becomes
 0. 3. The method of claim 2, wherein a length of thedefer duration is 25 us.
 4. The method of claim 1, wherein the SRS istransmitted in the last symbol among symbols included in a samesubframe.
 5. The method of claim 1, wherein a channel access priorityclass for the SRS transmission is configured to a channel accesspriority class
 1. 6. The method of claim 1, wherein the UE is capable ofoccupying the unlicensed band during maximum 2 ms based on the CAP forthe SRS transmission.
 7. A user equipment (UE) for transmitting asounding reference signal (SRS) to a base station in a wirelesscommunication system supporting an unlicensed band, the UE comprising: atransmitter; a receiver; at least one processor; and at least onecomputer memory operably connectable to the at least one processor andstoring instructions that, when executed by the at least one processor,perform operations comprising: performing a channel access procedure(CAP) for SRS transmission, transmitting the SRS via the unlicensed bandbased on the CAP, and wherein the CAP is a random backoff-based CAP anda predetermined channel access priority class has a value of {3,7} as anallowed contention window size, wherein, based on a contention windowsize most recently applied to the CAP for transmitting a physical uplinkshared channel (PUSCH) being a minimum contention window size, the value3 is applied as the contention window size applied to the CAP fortransmitting the SRS, and wherein, based on the contention window sizemost recently applied to the CAP for transmitting the PUSCH being anincreased window size relative to the minimum contention window size,the value 7 is applied as the contention window size applied to the CAPfor transmitting the SRS.
 8. The UE of claim 7, the random backoff-basedCAP comprising: randomly selecting a counter N between 0 and acontention window size for a SRS transmission, wherein the counter N isdecremented by 1 based on determination that the unlicensed band issensed to be idle during a defer duration; and determining that thecounter N becomes
 0. 9. The UE of claim 8, wherein a length of the deferduration is 25 us.
 10. The UE of claim 7, wherein the SRS is transmittedin the last symbol among symbols included in a same subframe.
 11. The UEof claim 7, wherein a channel access priority class for the SRStransmission without the PUSCH is configured to a channel accesspriority class
 1. 12. The UE of claim 7, wherein the UE is capable ofoccupying the unlicensed band during maximum 2 ms based on the CAP forthe SRS transmission.